This invention relates generally to pilot operated spool valve assemblies, and more particularly to hydraulically actuated fuel injectors that use such valves.
Hydraulically actuated fuel injectors are used in many internal combustion engines and have performed very well over the years. In these injectors, high pressure hydraulic oil is used to pressurize fuel for injection into the combustion space and also to control the opening and closing of valves within the injector body. In one class of hydraulically actuated fuel injectors, a solenoid-driven pilot valve controls the initiation of the injection event. One example of such a fuel injector is described in U.S. Pat. No. 5,682,858, issued to Chen et al. on Nov. 4, 1997. When the pilot valve is actuated, the pressure control passage defined by the valve body becomes fluidly connected to a low pressure vent. This sudden drop in pressure allows both the opening of a spring-biased direct control needle valve and the downward movement of a spring-biased spool valve member. When the spool moves to its downward position, it allows high pressure actuation fluid to drive an intensifier piston down, pressurizing fuel sufficiently to lift the needle valve and open the nozzle outlet. The use of an electronically controlled hydraulic system to inject fuel allows the timing and quantity of fuel injected to be precisely controlled, resulting in improved engine performance and better emissions.
The performance and efficiency levels reached with pilot operated spool valve assemblies are excellent. There is of course always room for improvement, especially under certain operating conditions. One development challenge in particular involves the displacement of cold hydraulic fluid from below the spool when the spool valve member travels downward at the initiation of an injection event. The plumbing in earlier injectors often required nearly full travel of the spool before start of injection could occur. During cold start, the hydraulic oil is particularly viscous, rendering it more difficult to displace through the relatively small drain path provided past the pilot valve member. This in turn can sometimes result in excessive spool travel times and correspondingly longer than desired start of current to start of injection times.
This slower spool valve response is a major factor in reducing the level of performance, resulting in difficulty achieving accurate start of injection timing (especially during cranking) and difficulty in reaching the higher end of the cold operating speed range. In earlier injectors of this type, such as that taught in Chen et al., the only path for draining the fluid beneath the spool was up the passage that controls check motion (the pressure control passage), and past the pilot stage lower seat. Therefore, a hydraulically actuated fuel injector including alternate means for evacuating hydraulic fluid could improve performance of hydraulically actuated fuel injectors, particularly at cold start.
The present invention is directed to overcoming one or more of the problems as set forth above.
In one aspect of the present invention, a pilot operated spool valve assembly includes a valve body defining a high pressure passage, a low pressure passage, a pressure control passage and a low pressure space. A spool valve member is movably positioned in the valve body and has a control hydraulic surface that is exposed to fluid pressure in the pressure control passage. A pilot valve member is positioned in the valve body and has a first position in which the high pressure passage is fluidly connected to the pressure control passage and a second position in which the low pressure passage is fluidly connected to the pressure control passage. A fluid evacuation valve member is positioned in the valve body and is movable between an open position in which the pressure control passage is fluidly connected to the low pressure space and a closed position.
In another aspect of the present invention, a hydraulic device includes a device body that defines a high pressure passage, a low pressure passage, a pressure control passage, and actuation fluid passage and a low pressure space. A spool valve member is positioned in the device body and has a control hydraulic surface that is exposed to fluid pressure in the pressure control passage. The spool valve member is movable between an on position in which the actuation fluid passage is open to the high pressure passage and an off position in which the actuation fluid passage is open to the low pressure passage. A pilot valve member is positioned in the device body that has a first position in which the high pressure passage is fluidly connected to the pressure control passage, and a second position in which the low pressure passage is fluidly connected to the pressure control passage. A fluid evacuation valve member is positioned in the device body and is movable between an open position in which the pressure control passage is fluidly connected to the low pressure space and a closed position. A piston is movably positioned in the device body and has a hydraulic surface that is exposed to fluid pressure in the actuation fluid passage.
In yet another aspect of the present invention, a method of operating a control valve includes providing a pilot operated spool valve assembly having a valve body that defines a high pressure passage and a low pressure passage, and has a spool valve member, a pilot valve member and a fluid evacuation valve member. The pilot valve member is moved to a first position to expose a control hydraulic surface of the spool valve member and a closing hydraulic surface of the fluid evacuation valve member to the low pressure passage. The spool valve member is then moved toward an on position to expose the fluid evacuation valve member to fluid pressure. Next the fluid evacuation valve member is moved to an open position. The pilot valve member is then moved to a second position to expose a control hydraulic surface of the spool valve member and a closing hydraulic surface of the fluid evacuation valve member to the high pressure passage. The spool valve member is moved toward an off position. The fluid evacuation valve member is moved to a closed position.